Wireless communication systems, i.e. systems that provide communication services to wireless communication devices such as mobile phones, smartphones (often denoted by UE that is short for user equipment) as well as machine-type communication (MTC) devices, have evolved during the last decade into systems that must utilize the radio spectrum and other system resources in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time, at any geographical location and also in scenarios where the wireless communication device is moving at a high speed, e.g., on board a high speed train.
To meet this demand, within the third generation partnership project (3GPP) work is being done regarding possible enhancements to radio resource management, RRM, performance in high speed train environments. The justification is that there are railways such as Japan Tohoku Shinkansen (running at 320 km/h), German ICE (330 km/h), AGV Italo (400 km/h), and Shanghai Maglev (430 km/h) at which vehicles travel at greater speed than 300 km/h and where there is demand for using mobile services.
For example, a new channel model for a particular high speed train (HST) scenario has been included in the study item technical report 3GPP RAN4 TR 36.878 V0.2.0. The scenario comprises cells consisting of multiple remote radio heads (RRHs) along the railway track, with downlink transmission (DLTX) antennas/radio lobes and uplink reception (ULRX) antennas/radio lobes, respectively, pointing towards each other.
FIG. 1 shows an example of an RRH arrangement for bidirectional RRH arrangement. In FIG. 1, an east moving UE 101 is onboard an east moving high speed train 102 on a railway track 131 and a west moving UE 103 is onboard a west moving high speed train 104 on a railway track 132. A first antenna node 110, which may be in the form of a RRH, maintains radio lobes including a transmission radio lobe 113, i.e. a DLTX lobe, and a reception radio lobe 114, i.e. an ULRX lobe. Similarly, a second antenna node 120 maintains radio lobes including a transmission radio lobe 121, i.e. a DLTX lobe, and a reception radio lobe 122, i.e. an ULRX lobe. As FIG. 1 illustrates, the transmission radio lobes 113, 121 of the respective antenna nodes 110, 120 are opposing each other and the reception radio lobes 114, 122 of the respective antenna nodes 110, 120 are opposing each other. FIG. 1 further illustrates a distance scale that shows a normalized distance measure, i.e. distance expressed as a percentage of the inter site distance (ISD), between the first antenna node 110 and the second antenna node 120. The east moving UE 101 is at a position that corresponds to 50% of the inter-site distance between the antenna nodes 110, 120.
This arrangement (in FIG. 1) is already in use by at least one large wireless communication system operator, and it has been observed that the performance is not as good as expected. The characteristics of this arrangement has been analyzed and presented in 3GPP tdoc R4-154516, and the root cause of the problems has been identified. In brief, it is related to fading caused by sending the same signal from two directions, and inter-carrier interference (ICI) due to different signs of the Doppler shifts experienced by the UE when receiving from the head or the tail direction with respect to the movement. Referring to FIG. 1, the east moving UE receives the DLTX lobe 121 from the head direction and the DLTX lobe 113 from the tail direction.
Fading and ICI significantly reduces the achievable system throughput for speeds up to 350 km/h (2.7 GHz band). Above 350 km/h the throughput for a legacy UE is less than 10% of that achievable by other RRH arrangements (see e.g. tdoc R4-154520). Since there are wireless communication systems having such bidirectional deployment already in use, there is a strong push in the standardization work to introduce UEs capable of achieving a better throughput by using advanced receiver techniques—one example can be found in 3GPP tdoc R4-154243 where a high-speed enabled UE (“HeUE”) is proposed. This “high-speed enabled” UE is supposed to take the bidirectional RRH deployment into account e.g. in channel estimation to thereby improve the performance.
However, there are problems with existing solutions. The harsh reality is that even if a new UE type that is able to achieve higher throughput in bidirectional RRH arrangements is introduced (earliest from 3GPP Release13), legacy UEs (up to 3GPP Release12) will be very common for years to come.
Consequently, unless it is taken into account in the scheduling whether a UE is of legacy type or of high-speed enabled type, not much improvement will be seen on the system capacity. This is so since the legacy UEs will consume a large part of the system capacity on more robust transmission (lower coding and modulation schemes), more retransmissions, and more overhead from radio link failure (RLF)-related signaling.